High pressure discharge lamps of elevated power, particularly discharge lamps retaining a metal halide fill, as well as xenon high pressure discharge lamps and mercury vapor high pressure discharge lamps are used, for example, in film and television studio illumination; some of those lamps are particularly adapted to simulate daylight or sunlight. Metal halide discharge lamps, for example, are formed with relatively long neck portions extending from a discharge bulb. The neck portions are melt-sealed to the discharge bulb. The relatively long neck portions permit locating of the sealing foils remote from the discharge arc within the bulb, and thus avoid difficulties which arise, in operation of the lamp, with respect to sealing of the discharge bulb due to the heat of the discharge arc between the electrodes. During the melt-sealing of the sealing foils in the electrode shafts, the quartz glass of the discharge bulb, and particularly quartz glass material adjacent the neck portions, must not touch the electrode shafts. The electrode shafts, typically made of tungsten, have a thermal coefficient of expansion which differs substantially from that of quartz glass. Upon cooling of the neck portions, immediately after sealing, substantial mechanical stresses may occur between the tungsten electrode shaft and the quartz glass, which lead to fissures and cracks in the quartz glass and either a reject of the lamp, or, later on, premature failure of the lamp.
It has previously been proposed--see the referenced U.S. Pat. No. 3,742,283, Loughridge, to surround the electrode shafts in the region of a pinch seal with concentric tubes of Cermet, to reduce the mechanical stresses in the region of the pinch seal. Cermet is a melt connection or alloy of a pulverized metal and quartz glass. The thermal coefficient of expansion of Cermet is between that of quartz glass and of the electrode shaft. Cermet tubes are not suitable for elevated power high pressure discharge lamps since, in high-power discharge lamps, the electrode shafts during the melt sealing of the sealing foils are subjected to temperatures of a level which the Cermet tubes cannot accept.
British patent 1,515,583, Beeson et al, proposed to wrap the electrode shafts with windings of molybdenum wire or tungsten wire in order to prevent adhesion of molten quartz glass to the electrode shaft. It is very difficult and expensive to manufacture suitable wire windings and apply such wire windings to the electrode shaft.